In the packaging of certain types of foods, such as cookies, potato chips, and the like, it is common practice to employ a multilayer film having two or more polymeric layers wherein one of the layers is known to be an effective heat seal layer. In the packaging process, a supply of such a multilayer film can be shaped into a tube in a vertical form and fill machine. Marginal regions of the heat seal layer are brought into face to face relationship and heat sealed together. Thereafter, the packaging machine automatically forms a heat seal and makes a horizontal severance across the bottom of the bag. Next, product is dispensed into the open end of the tube and, thereafter, a second horizontal seal is effected across the tube with a simultaneous severing through the tube to result in a product packaged in a tube, heat sealed at both ends and along one seam at right angles to the end seals. While the food or other product is being dispensed into the package, air is also present in the package and this air assists in protecting and cushioning the product during subsequent shipment of the finished packages. During shipment of the product, particularly with larger size bags, e.g., those containing 6 ounces of product, the bags have a tendency to split or burst at the end seals.
The employment of vinylidene chloride polymers as thin coatings for oriented thermoplastic films such as polypropylene film has been recognized in the art as an effective means of increasing the heat sealability of such films and additionally increasing the resistance of the films to the transmission of gases and vapors. This latter characteristic is particularly useful when such films are employed to package products which are sensitive to, or attacked by, oxygen or moisture.
A multilayer wrapping film of the type having the above-mentioned utility is described in U.S. Pat. No. 4,214,039, the subject matter of which is, in its entirety, incorporated by reference herein. U.S. Pat. No. 4,214,039 describes a packaging film structure comprising a polypropylene film substrate having a heat sealable vinylidene chloride polymer containing at least 50 percent by weight of vinylidene chloride. In between the polypropylene film and the vinylidene chloride polymer is a primer coat which consists of the reaction product of an acidified aminoethylated vinyl polymer and an epoxy resin. This primer coat enhances the bond strength between the polypropylene and the heat sealable vinylidene chloride polymer.
U.S. Pat. No. 4,439,493 discloses an oriented heat sealable structure which comprises a polyolefin film substrate, a layer consisting essentially of a random copolymer of ethylene and propylene having from about 0.5% to about 6% by weight of ethylene on at least one surface of the substrate, a primer coating on at least one surface of the random copolymeric layer and a heat sealable layer on the primer coating, wherein the heat sealable layer comprises an interpolymer comprising a minor amount of acrylic acid, methacrylic acid or mixtures thereof and a minor amount of neutral monomer esters comprising methyl acrylate, ethyl acrylate or methyl methacrylate. U.S. Pat. No. 4,439,493 is incorporated by reference in its entirety for all that it discloses.
U.S. Pat. No. 4,447,494, incorporated by reference in its entirety, discloses another oriented heat sealable structure, this one comprising a polyolefin film substrate, a layer consisting essentially of a random copolymer of ethylene and propylene having from about 0.5% to about 6% by weight of ethylene on at least one surface of the substrate, a primer coating on at least one surface of the random copolymeric layer and a heat sealable layer on the primer coating, wherein the heat sealable layer comprises a vinylidene chloride copolymer containing at least 50% by weight of vinylidene chloride.
U.S. Pat. No. 4,564,558 discloses a multilayer oriented heat sealable structure, comprising a polyolefin film substrate, a layer comprising a terpolymer of propylene with ethylene and butene-1, a primer coating on at least one surface of the terpolymer layer and a heat sealable layer on the primer coating, wherein the heat sealable layer is selected from the group consisting of a vinylidene chloride polymer layer and an acrylic polymer layer. U.S. Pat. No. 4,564,558 is incorporated by reference in its entirety for all that it discloses.
Despite these advances in the art, there exists a need for other multilayer heat sealable structures having improved properties.